Inkjet ink set for preparing conductive layers or patterns

ABSTRACT

An inkjet ink set including a silver inkjet ink and a flushing liquid, characterized in that the flushing liquid comprises at least 25 wt % of 2-phenoxy ethanol based on the total weight of the flushing liquid.

FIELD OF THE INVENTION

The present invention relates to an inkjet ink set for preparingconductive layers or patterns, in particular conductive silver layers orpatterns.

The invention also relates to a method of preparing the conductivelayers or patterns at moderate curing conditions.

BACKGROUND OF THE INVENTION

The interest in metallic printing or coating fluids comprising metallicnanoparticles has increased during the last decades due to their uniqueproperties when compared to the bulk properties of a given metal. Forexample, the melting point of metallic nanoparticles decreases withdecreasing particle size making them of interest for printedelectronics, electrochemical, optical, magnetic and biologicalapplications.

The production of stable and concentrated metallic printing or coatingfluids which can be printed, for example by inkjet printing, or coatedat high speed is of great interest as it enables the preparation ofelectronic devices at low costs.

Metallic printing or coating fluids are typically a metallicnanoparticle dispersion comprising metallic nanoparticles and adispersion medium. Such metallic nanoparticle dispersions can bedirectly used as a printing or coating fluid. However, additionalingredients are often added to the metallic nanoparticle dispersion tooptimize the properties of the resulting metallic printing or coatingfluids.

Typically, after applying the metallic printing or coating fluids on asubstrate, a sintering step, also referred to as curing step, atelevated temperatures is carried out to induce/enhance the conductivityof the applied patterns of layers. The organic components of themetallic printing or coating fluids, for example the polymericdispersants, may reduce the sintering efficiency and thus theconductivity of the applied patterns of layers. For this reason, highersintering temperatures and longer sintering times are often required todecompose the organic components.

EP-A 2671927 discloses a metallic nanoparticle dispersion, for example asilver inkjet ink, comprising a specific dispersion medium, for example2-pyrrolidone, resulting in a more stable dispersion without using apolymeric dispersant.

Unpublished EP-A 14199745.2 (filed 22 Dec. 2014) discloses a metallicnanoparticle dispersion comprising silver nanoparticles, a liquidcarrier and specific dispersion stabilizing compounds.

A problem often encountered when using a silver inkjet ink is so-calledclogging of the inkjet printheads. Such clogging of the inkjetprintheads may result in printing defects and may shorten the lifetimeof the printhead.

It is known to use so-called “flushing” or “washing” liquids forunclogging inkjet nozzles and cleaning the nozzle plate of the printhead, as well as all the tubing and connections between the ink tank tothe printhead. Such flushing liquids typically comprise of a solvent ora solvent mixture able to remove efficiently all inkjet ink residueswithout affecting the stability of the printhead. Additionally, theflushing solution must be also compatible in all its proportions withthe employed inkjet ink.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an inkjet ink setfor preparing conductive layers or patterns, in particular conductivesilver layers or patterns, whereby no or a minimal amount of printingdefects are observed and whereby the lifetime of the inkjet printheadsis improved.

This object is realized by an inkjet ink set including a silver inkjetink and a flushing liquid as defined in claim 1.

The invention also relates to a method of preparing the conductivelayers or patterns at moderate curing conditions using the inkjet inkset.

Further advantages and embodiments of the present invention will becomeapparent from the following description and the dependent claims.

DETAILED DESCRIPTION OF THE INVENTION Definitions

The terms polymeric support and foil, as used herein, mean aself-supporting polymer-based sheet, which may be associated with one ormore adhesion layers, e.g. subbing layers. Supports and foils areusually manufactured through extrusion.

The term layer as used herein, is considered not to be self-supportingand is manufactured by coating or spraying it on a (polymeric) supportor foil.

PET is an abbreviation for polyethylene terephthalate.

The term alkyl means all variants possible for each number of carbonatoms in the alkyl group i.e. methyl, ethyl, for three carbon atoms:n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl andtertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl,2,2-dimethylpropyl and 2-methyl-butyl etc.

Unless otherwise specified a substituted or unsubstituted alkyl group ispreferably a C₁ to C₆-alkyl group.

Unless otherwise specified a substituted or unsubstituted alkenyl groupis preferably a C₂ to C₆-alkenyl group.

Unless otherwise specified a substituted or unsubstituted alkynyl groupis preferably a C₂ to C₆-alkynyl group.

Unless otherwise specified a substituted or unsubstituted aralkyl groupis preferably a phenyl group or a naphthyl group including one, two,three or more C₁ to C₆-alkyl groups.

Unless otherwise specified a substituted or unsubstituted alkaryl groupis preferably a C₁ to C₆-alkyl group including an aryl group, preferablya phenyl group or naphthyl group.

Unless otherwise specified a substituted or unsubstituted aryl group ispreferably a substituted or unsubstituted phenyl group or naphthylgroup.

A cyclic group includes at least one ring structure and may be amonocyclic- or polycyclic group, meaning one or more rings fusedtogether.

A heterocyclic group is a cyclic group that has atoms of at least twodifferent elements as members of its ring(s). The counterparts ofheterocyclic groups are homocyclic groups, the ring structures of whichare made of carbon only. Unless otherwise specified a substituted orunsubstituted heterocyclic group is preferably a five- or six-memberedring substituted by one, two, three or four heteroatoms, preferablyselected from oxygen atoms, nitrogen atoms, sulphur atoms, seleniumatoms or combinations thereof.

An alicyclic group is a non-aromatic homocyclic group wherein the ringatoms consist of carbon atoms.

The term heteroaryl group means a monocyclic- or polycyclic aromaticring comprising carbon atoms and one or more heteroatoms in the ringstructure, preferably, 1 to 4 heteroatoms, independently selected fromnitrogen, oxygen, selenium and sulphur. Preferred examples of heteroarylgroups include, but are not limited to, pyridinyl, pyridazinyl,pyrimidyl, pyrazyl, triazinyl, pyrrolyl, pyrazolyl, imidazolyl, (1,2,3)-and (1,2,4)-triazolyl, pyrazinyl, pyrimidinyl, tetrazolyl, furyl,thienyl, isoxazolyl, thiazolyl, isoxazolyl, and oxazolyl. A heteroarylgroup can be unsubstituted or substituted with one, two or more suitablesubstituents. Preferably, a heteroaryl group is a monocyclic ring,wherein the ring comprises 1 to 5 carbon atoms and 1 to 4 heteroatoms.

The term substituted, in e.g. substituted alkyl group means that thealkyl group may be substituted by other atoms than the atoms normallypresent in such a group, i.e. carbon and hydrogen. For example, asubstituted alkyl group may include a halogen atom or a thiol group. Anunsubstituted alkyl group contains only carbon and hydrogen atoms.

Unless otherwise specified a substituted alkyl group, a substitutedalkenyl group, a substituted alkynyl group, a substituted aralkyl group,a substituted alkaryl group, a substituted aryl, a substitutedheteroaryl and a substituted heterocyclic group are preferablysubstituted by one or more substituents selected from the groupconsisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, 1-isobutyl,2-isobutyl and tertiary-butyl, ester, amide, ether, thioether, ketone,aldehyde, sulfoxide, sulfone, sulfonate ester, sulphonamide, —Cl, —Br,—I, —OH, —SH, —CN and —NO₂.

Inkjet Ink Set.

The inkjet ink set includes a silver inkjet ink and a flushing liquid,wherein the flushing liquid comprises at least 25 wt % of 2-phenoxyethanol based on the total weight of the flushing liquid.

Flushing Liquid

The flushing liquid comprises at least 25 wt %, preferably at least 40wt %, more preferably at least 50 wt % of 2-phenoxy ethanol based on thetotal weight of the flushing liquid.

The flusing liquid may comprise another solvent, preferably a highboiling solvent.

High boiling organic solvents referred to herein are solvents which havea boiling point that is higher than the boiling point of water (>100°C.).

Preferred high boiling solvents are shown in Table 1.

TABLE 1 Chemical formula Chemical name Bp (° C.)

4-methyl-1,3-dioxolan-2-one (propylene carbonate) 242

n-butanol 117

1,2-propanediol 211-217

4-hydroxy-4-methylpentan-2-one (diaceton alcohol) 168

Pentan-3-one (diethyl ketone) 102

2-Butoxyethanol Ethylene glycol monobutyl ether 171

Dihydrofuran-2(3H)-one (Gamma-butyrolacton) 204

2-pyrrolidon 245

1-methoxy-2-propanol (propyleneglycolmonomethylether 120

The flushing liquid preferably comprises at least 25 wt % of2-phenoxyethanol and a further solvent selected from the groupconsisting of propylene carbonate, n-butanol and 2-pyrrolidone.

A particularly preferred flushing liquid comprised at least 25 wt % of2-phenoxy ethanol and from 5 wt % to 20 wt % of n-butanol, all based onthe total weight of the Flushing liquid.

The viscosity at 25° C. of the flushing liquid preferably is preferablylower than 20 mPa·s, more preferably less than 15, most preferably lessthan 10 mPa·s.

Silver Inkjet Ink

The silver inkjet ink preferably comprises silver nanoparticles, aliquid carrier and a dispersion-stabilizing compound (DSC).

The silver inkjet ink may further comprise a polymeric dispersant andadditives to further optimize its properties.

Dispersion-Stabilizing Compound (DSC)

The silver inkjet ink preferably comprises silver nanoparticles, aliquid carrier and a dispersion-stabilizing compound (DSC) according toFormulae I, II, III or IV,

whereinQ represents the necessary atoms to form a substituted or unsubstitutedfive or six membered heteroaromatic ring;M is selected from the group consisting of a hydrogen, a monovalentcationic group and an acyl group;R1 and R2 are independently selected from the group consisting of ahydrogen, a substituted or unsubstituted alkyl group, a substituted orunsubstituted alkenyl group, a substituted or unsubstituted alkynylgroup, a substituted or unsubstituted alkaryl group, a substituted orunsubstituted aralkyl group, a substituted or unsubstituted aryl orheteroaryl group, a hydroxyl group, a thioether, an ether, an ester, anamide, an amine, a halogen, a ketone and an aldehyde;R1 and R2 may represent the necessary atoms to form a five to sevenmembered ring;R3 to R5 are independently selected from the group consisting of ahydrogen, a substituted or unsubstituted alkyl group, a substituted orunsubstituted alkenyl group, a substituted or unsubstituted alkynylgroup, a substituted or unsubstituted alkaryl group, a substituted orunsubstituted aralkyl group, a substituted or unsubstituted aryl orheteroaryl group, a hydroxyl group, a thiol, a thioether, a sulfone, asulfoxide, an ether, an ester, an amide, an amine, a halogen, a ketone,an aldehyde, a nitrile and a nitro group; R4 and R5 may represent thenecessary atoms to form a five to seven membered ring.

The dispersion-stabilizing compound is preferably a compound accordingto Formula I.

The dispersion-stabilizing compound is more preferably a compoundaccording to Formula I, wherein Q represents the necessary atoms to forma five membered heteroaromatic ring.

A particular preferred dispersion-stabilizing compound is a compoundaccording Formula I, wherein Q is a five membered heteroaromatic ringselected from the group consisting of an imidazole, a benzimidazole, athiazole, a benzothiazole, an oxazole, a benzoxazole, a 1,2,3-triazole,a 1,2,4-triazole, an oxadiazole, a thiadiazole and a tetrazole.

Some examples of dispersion-stabilizing compounds according to thepresent invention are shown in the following table.

DSC Chemical Formula DCS-01

DCS-02

DCS-03

DCS-04

DCS-05

DCS-06

DCS-07

DCS-08

DCS-09

DCS-10

DCS-11

DCS-12

DCS-13

DCS-14

DCS-15

DCS-16

The dispersion-stabilizing compound is preferably selected from thegroup consisting ofN,N-dibutyl-(2,5-dihydro-5-thioxo-1H-tetrazol-1-yl-acetamide,5-heptyl-2-mercapto-1,3,4-oxadiazole, 1-phenyl-5-mercaptotetrazol,5-methyl-1,2,4-triazolo-(1,5-a) primidine-7-ol, andS[5-[(ethoxycarbonyl)amino]-1,3,4-thiadiazol-2-yl] O-ethylthiocarbonate.

The dispersion-stabilizing compounds according to Formula I to IV arepreferably non-polymeric compounds. Non-polymeric compounds as usedherein means compounds having a Molecular Weight which is lesspreferably than 1000, more preferably less than 500, most preferablyless than 350.

The amount of the dispersion-stabilizing compounds (DSC) expressed as wt% relative to the total weight of silver (Ag) in the silver inkjet inkis preferably between 0.005 and 10.0, more preferably between 0.0075 and5.0, most preferably between 0.01 and 2.5. When the amount of thedispersion-stabilizing compound relative to the total weight of silveris too low, the stabilizing effect may be too low, while a too highamount of the dispersion-stabilizing compound may adversely affect theconductivity of the coating or patterns obtained with the silver inkjetink.

Silver Nanoparticles

The dispersed silver nanoparticles have an average particle size oraverage particle diameter, measured with Transmission ElectronMicroscopy, of less than 150 nm, preferably less than 100 nm, morepreferably less than 50 nm, most preferably less than 30 nm.

The amount of silver nanoparticles in the inkjet is preferably at least5 wt %, more preferably at least 10 wt %, most preferably at least 15 wt%, particularly preferred at least 20 wt %, relative to the total weightof the silver inkjet ink.

The silver nanoparticles are preferably prepared by the method disclosedin EP-A 2671927, paragraphs [0044] to [0053] and the examples.

Polymeric Dispersant

The silver inkjet ink may contain a polymeric dispersant.

Polymeric dispersants typically contain in one part of the moleculeso-called anchor groups, which adsorb onto the silver particles to bedispersed. In another part of the molecule, polymeric dispersants havepolymer chains compatible with the dispersion medium, also referred toas liquid vehicle, and all the ingredients present in the final printingor coating fluids.

Polymeric dispersants are typically homo- or copolymers prepared fromacrylic acid, methacrylic acid, vinyl pyrrolidinone, vinyl butyral,vinyl acetate or vinyl alcohol monomers.

The polymeric dispersants disclosed in EP-A 2468827, having a 95 wt %decomposition at a temperature below 300° C. as measured by ThermalGravimetric Analysis may also be used.

However, in a preferred embodiment the silver inkjet ink comprises lessthan 5 wt % of a polymeric dispersant relative to the total weight ofthe dispersion, more preferably less than 1 wt %, most preferably lessthan 0.1 wt %. In a particularly preferred embodiment the dispersioncomprises no polymeric dispersant at all.

Liquid Carrier

The silver inkjet ink comprises a liquid carrier.

The liquid carrier is preferably an organic solvent. The organic solventmay be selected from alcohols, aromatic hydrocarbons, ketones, esters,aliphatic hydrocarbons, higher fatty acids, carbitols, cellosolves, andhigher fatty acid esters.

Suitable alcohols include methanol, ethanol, propanol, 1-butanol,1-pentanol, 2-butanol, t-butanol.

Suitable aromatic hydrocarbons include toluene and xylene.

Suitable ketones include methyl ethyl ketone, methyl isobutyl ketone,2,4-pentanedione and hexa-fluoroacetone.

Also glycol, glycolethers, N,N-dimethyl-acetamide, N,N-dimethylformamidemay be used.

A mixture of organic solvents may be used to optimize the properties ofthe metallic nanoparticle dispersion.

Preferred organic solvents are high boiling solvents. High boilingorganic solvents referred to herein are solvents which have a boilingpoint that is higher than the boiling point of water (>100° C.).

Preferred high boiling solvents are shown in Table 2.

TABLE 2 Chemical formula Chemical name Bp (° C.)

2-phenoxy ethanol (ethylene glycol monophenylether) 247

4-methyl-1,3-dioxolan-2-one (propylene carbonate) 242

n-butanol 117

1,2-propanediol 211-217

4-hydroxy-4- methylpentan-2-one (diaceton alcohol) 168

Pentan-3-one (diethyl ketone) 102

2-Butoxyethanol Ethylene glycol monobutyl ether 171

Dihydrofuran-2(3H)-one (Gamma-butyrolacton) 204

2-pyrrolidin 245

1-methoxy-2-propanol (propyleneglycol- monomethylether 120

Particularly preferred high boiling solvents are 2-phenoxy ethanol,propylene carbonate, propylene glycol, n-butanol, 2-pyrrolidone andmixtures thereof.

The silver ink preferably comprises at least 25 wt % of2-phenoxyethanol, more preferably at least 40 wt %, based on the totalweight of the silver ink.

Additives

To optimize the printing properties, and also depending on theapplication for which it is used, additives such as reducing agents,wetting/levelling agents, dewettting agents, rheology modifiers,adhesion agents, tackifiers, humectants, jetting agents, curing agents,biocides or antioxidants may be added to the silver inkjet ink describedabove.

The silver inkjet ink may comprise a surfactant. Preferred surfactantsare Byk® 410 and 411, both solutions of a modified urea, and Byk® 430, asolution of a high molecular urea modified medium polar polyamide.

The amount of the surfactants is preferably between 0.01 and 10 wt %,more preferably between 0.05 and 5 wt %, most preferably between 0.1 and0.5 wt %, relative to the total amount of the metallic nanoparticledispersion.

It may be advantageous to add a small amount of a metal of an inorganicacid or a compound capable of generating such an acid during curing of ametallic layer or pattern formed from the silver inkjet ink such asdisclosed in EP-A 2821164. Higher conductivities and/or lower curingtemperatures were observed of layers or patterns formed from such silverinkjet ink.

Higher conductivities and/or lower curing temperatures may also beobtained when using silver inkjet ink containing a compound according toFormula X, as disclosed in EP-A 3016763.

-   -   wherein    -   X represents the necessary atoms to form a substituted or        unsubstituted ring.

A particularly preferred compound according to Formula X is an ascorbicor erythorbic acid derivative compound.

Preparation of the Silver Inkjet Ink

The preparation of the silver inkjet ink according to the presentinvention typically comprises the addition of the liquid carrier, thedispersion-stabilizing compound and optional additives to the silvernanoparticles by using a homogenization technique such as stirring, highshear mixing, ultra-sonication, or a combination thereof.

The silver nanoparticles from which the silver inkjet ink is made istypically a paste or a highly concentrated dispersion of silvernanoparticles. A preferred preparation method of the metallicnanoparticles is disclosed in EP-A 2671927.

It has been observed that better results are obtained when all, or aportion, of the dispersion-stabilizing compound are added during thepreparation method of the silver nanoparticles. Due to their adsorptionto the silver nanoparticles, the dispersion-stabilizing compounds addedduring the preparation of the silver nanoparticles will be retained, atleast partially, in the final silver nanoparticle disperision, even ifone or more washing steps have been carried out in the preparationmethod.

The homogenization step can be carried out at elevated temperature up to100° C. In a preferred embodiment, the homogenization step is carriedout at temperature equal or below 60° C.

Silver Layers or Patterns

Silver layers or patterns may be printed with the silver inkjet ink.

Conductive silver layers or patterns are prepared by an inkjet printingmethod comprising the steps of jetting the silver inkjet ink on asupport followed by a curing step. Such a curing step is also referredto as a sintering step.

The support may be a glass, a paper or a polymeric support.

Preferred polymeric supports are polycarbonate, polyethyleneterephthalate (PET) or polyvinylchloride (PVC) based supports.

The above mentioned supports may be provided with one or more layers toimprove the adhesion, absorption or spreading of the applied conductiveinkjet inks.

Polymeric supports are preferably provided with so-called subbing layersto improve the adhesion of the applied conductive inkjet or flexo inks.Such subbing layers are typically based on vinylidene copolymers,polyesters, or (meth)acrylates.

Useful subbing layers for this purpose are well known in the art andinclude, for example, polymers of vinylidene chloride such as vinylidenechloride/acrylonitrile/acrylic acid terpolymers or vinylidenechloride/methyl acrylate/itaconic acid terpolymers.

Suitable vinylidene chloride copolymers include: the copolymer ofvinylidene chloride, N-tert.-butylacrylamide, n-butyl acrylate, andN-vinyl pyrrolidone (e.g. 70:23:3:4), the copolymer of vinylidenechloride, N-tert.-butylacrylamide, n-butyl acrylate, and itaconic acid(e.g. 70:21:5:2), the copolymer of vinylidene chloride,N-tert.-butylacrylamide, and itaconic acid (e.g. 88:10:2), the copolymerof vinylidene chloride, n-butylmaleimide, and itaconic acid (e.g.90:8:2), the copolymer of vinyl chloride, vinylidene chloride, andmethacrylic acid (e.g. 65:30:5), the copolymer of vinylidene chloride,vinyl chloride, and itaconic acid (e.g. 70:26:4), the copolymer of vinylchloride, n-butyl acrylate, and itaconic acid (e.g. 66:30:4), thecopolymer of vinylidene chloride, n-butyl acrylate, and itaconic acid(e.g. 80:18:2), the copolymer of vinylidene chloride, methyl acrylate,and itaconic acid (e.g. 90:8:2), the copolymer of vinyl chloride,vinylidene chloride, N-tert.-butylacrylamide, and itaconic acid (e.g.50:30:18:2). All the ratios given between brackets in theabove-mentioned copolymers are ratios by weight.

Other preferred subbing layers include a binder based on apolyester-urethane copolymer. In a more preferred embodiment, thepolyester-urethane copolymer is an ionomer type polyester urethane,preferably using polyester segments based on terephthalic acid andethylene glycol and hexamethylene diisocyanate. A suitablepolyester-urethane copolymer is Hydran™ APX101 H from DIC Europe GmbH.

The application of subbing layers is well-known in the art ofmanufacturing polyester supports for silver halide photographic films.For example, the preparation of such subbing layers is disclosed in U.S.Pat. No. 3,649,336 and GB 1441591.

In a preferred embodiment, the subbing layer has a dry thickness of nomore than 0.2 μm or preferably no more than 200 mg/m².

Another preferred support is an ITO based support. Such a support istypically a glass or polymer support whereupon an ITO layer or patternis provided.

A preferred paper based support is the Powercoat® paper substrate, asubstrate designed for printed electronics by Arjowiggins CreativePapers.

Multiple silver layers or patterns, i.e. a stack of patterned orunpatterned layers, may be applied on a substrate. The support referredto in the method of preparing the silver layers or patterns thus alsoencompass a previously applied silver layer or pattern.

An inkjet printing method of preparing a conductive layer or patterncomprising the steps of:

-   -   cleaning the print head with a flushing liquid as described        above prior to load the Ag inkjet ink,    -   jetting a silver inkjet ink with an inkjet printer comprising a        print head on a support thereby forming a silver layer or        pattern on the support;    -   curing the silver layer or pattern thereby forming a conductive        layer or pattern; and    -   cleaning the print head with a flushing liquid as described        above.

The silver inkjet ink is preferably as described above.

In a preferred inkjet printing method the curing temperature is below150° C. and the curing time is less than 30 minutes.

Inkjet Printing Devices

Various embodiments of an apparatus for creating silver layers orpattern from the silver inkjet inks according to the present inventionby inkjet printing may be used.

In a flat bed printing device a support is provided on a flat bed.Droplets of a silver inkjet fluid are jetted from a print head on thesupport.

The print heads typically scan back and forth in a transversal direction(x-direction) across a moving support (y-direction). Such bi-directionalprinting is referred to as multi-pass printing.

Another preferred printing method is the so-called single-pass printingmethod wherein the print heads, or multiple staggered print heads, coverthe entire width of the support. In such a single-pass printing method,the print heads usually remain stationary while the support istransported under the print heads (y-direction).

To obtain maximal dot placement accuracy, the print heads are positionedas close as possible to the surface of the support. The distance betweenthe print heads and the surface of the support is preferably less than 3mm, more preferably less than 2 mm, most preferably less than 1 mm.

As the distance between the printhead and the surface of the support mayinfluence the dot placement accuracy, it may be advantageous to measurethe thickness of a support and adapting the distance between theprinthead and the surface of the support based on the measurement of thethickness of the support.

The distance between a stationary printhead and the surface of a supportmounted on the printing device may also vary over the whole support, dueto for example waviness of the support, or other irregularities in thesurface of the support. Therefore it may also be advantageous to measurethe surface topography of the support and to compensate the differencesin the measured surface topography by controlling the so-called firingtime of the droplets of curable fluids on the support, or by adjustingthe distance between the printhead and the surface of the support.Examples of measurement devices to measure the surface topography of alithographic supports is disclosed in ISO 12635:2008(E).

In a preferred embodiment the inkjet printing device has holding downmeans, such as a vacuum chamber under the support, to hold down thesupport in a so-called hold-down zone, for example by vacuum. In a morepreferred embodiment the support is hold down against the support byindependent working holding down means such as a plurality of vacuumchambers under the support which are independently controlled to enhancethe vacuum pressure on the support so that more than one hold down zonesare generated on the support. The holding down of the support enhancesthe drop placement of the jetted droplets and position accuracy.

Print Head

A preferred print head for the inkjet printing system is a piezoelectrichead.

Piezoelectric inkjet printing is based on the movement of apiezoelectric ceramic transducer when a voltage is applied thereto. Theapplication of a voltage changes the shape of the piezoelectric ceramictransducer in the print head creating a void, which is then filled withink. When the voltage is again removed, the ceramic expands to itsoriginal shape, ejecting a drop of ink from the print head. However theinkjet printing method according to the present invention is notrestricted to piezoelectric inkjet printing. Other inkjet print headscan be used and include various types, such as the continuous printingtype.

Preferred print heads eject droplets having a volume ≤50 pL, morepreferably ≤35 pL, most preferably ≤25 pL, particularly preferred ≤15pL.

Another preferred print head is a throughflow piezoelectric inkjet printhead. A throughflow piezoelectric inkjet print head is a print headwherein a continuous flow of liquid is circulating through the liquidchannels of the print head to avoid agglomerations in the liquid whichmay cause disturbing effects in the flow and bad drop placements.Avoiding bad drop placements by using throughflow piezoelectric inkjetprint heads may improve the quality of the conductive patterns on thesupport. Another advantage of using such throughflow print heads is ahigher viscosity limit of the curable fluids to be jetted, widening thescope of compositional variations of the fluids.

Curing Step

After the layers or patterns are applied on the support, a sinteringstep, also referred to as curing step, is carried out. During thissintering step, solvents evaporate and the silver particles sintertogether. Once a continuous percolating network is formed between themetallic particles, the layers or patterns become conductive.Conventional curing is typically carried out by applying heat. Thecuring temperature and time are dependent on the support used and on thecomposition of the metallic layer or pattern. The curing step for curingthe silver layers may be performed at a temperature below 200° C.,preferably below 180° C., more preferably below 150° C., most preferablybelow 130° C.

The curing time may be less than 60 minutes, preferably between 2 and 30minutes and more preferably between 3 and 20 minutes, depending on theselected temperature, support and composition of the metallic layers.

However, instead of or in addition to the conventional sintering byapplying heat, alternative sintering methods such as exposure to anArgon laser, to microwave radiation, to UV radiation or to low pressureArgon plasma, photonic curing, plasma or plasma enhanced, electron beam,laser beam or pulse electric current sintering may be used.

Another curing method uses the so-called Near infrared (NIR) curingtechnology. The metal of the coating or the pattern, for example silver,may act as absorber for the NIR radiation.

The silver layers of the present invention allow to use lower curingtemperatures than the prior art processes. In consequence it is possibleto use polymeric substrates that can not withstand thermal treatment athigh temperature, such as for example PET. The curing time may also besubstantially reduced leading to the possibility of having higherproduction per hour than the prior art processes. The conductivity ofthe silver layers are maintained or even improved in certain cases.

To further increase the conductivity or to lower the curing temperatureit may be advantageous to contact the silver layer or pattern with asolution containing an acid or an acid precursor capable or releasingthe acid during curing of the metallic layer or pattern, as disclosed inEP-A 13175030.9 (filed on Apr. 7, 2013).

The silver layers or patterns may be used in various electronic devicesor parts of such electronic devices as for example organicphoto-voltaics (OPV's), inorganic photo-voltaics (c-Si, a-Si, CdTe,CIGS), OLED displays, OLED lighting, inorganic lighting, RFID's, organictransistors, thin film batteries, touch-screens, e-paper, LCD's, plasma,sensors, membrane switches or electromagnetic shielding.

Additionally, the silver layers or patterns may be used in varioussecurity or decorative devices or parts of such security or decorativedevices, for example when an image with special features that willimpede its complete or partial reproduction or a light reflective layerwith metal appearance is required.

EXAMPLES

Materials

All materials used in the following examples were readily available fromstandard sources such as ALDRICH CHEMICAL Co. (Belgium) and ACROS(Belgium) unless otherwise specified. The water used was deionizedwater.

DSC-01 is the dispersion-stabilizing compoundN-dibutyl-(2,5-dihydro-5-thioxo-1H-tetrazol-1-yl)acetamide(CASRN168612-06-4) commercially available from Chemosyntha.

Silver oxide, Ag₂O commercially available from UMICORE.

2-phenoxy-ethanol (CASRN122-99-6) commercially available from BASF.

Gamma-butyro-lactone (CASRN96-48-0) commercially available from BASF.

Propylenecarbonate (CASRN108-32-7) commercially available from SigmaAldrich.

Diaceton alcohol (CASRN123-42-2) commercially available from ACROSCHIMICA.

n-butanol (CASRN71-36-3) commercially available from ACROS CHIMICA.

1,2 propanediol (CASRN57-55-6) commercially available from ACROSCHIMICA.

1-methoxy-2-propanol (CASRN107-98-2) commercially available from DOWCHEMICALS.

2-butoxyethanol (CASRN111-76-2) commercially available from DOWCHEMICALS. Copol (ViCl₂-MA-IA), a copolymer ofvinylidenechloride-methacrylic acid and itaconic acid from Agfa Gevaert.

Mersolat H40, a surfactant from Lanxess.

Kieselsol 100F, a silica from Bayer.

Measurements Methods

Conductivity of the Silver Coatings

The surface resistance (SER) of the silver coatings was measured using afour-point collinear probe. The surface or sheet resistance wascalculated by the following formula:

SER=(π/ln 2)*(V/I)

whereinSER is the surface resistance of the layer expressed in Ω/square;π is a mathematical constant, approximately equal to 3.14;ln 2 is a mathematical constant equal to the natural logarithmic ofvalue 2, approximately equal to 0.693;V is voltage measured by voltmeter of the four-point probe measurementdevice;I is the source current measured by the four-point probe measurementdevice.

For each sample, six measurements were performed at different positionsof the coating and the average value was calculated.

The silver content M_(Ag) (g/m²) of the coatings was determined byWD-XRF.

The conductivity of the coated layers was then determined by calculatingthe conductivity as a percentage of the bulk conductivity of silverusing the following formula:

${\%{Ag}_{({bulk})}} = {\frac{\sigma_{Coat}}{\sigma_{Ag}} \times 100}$${\%{Ag}_{({bulk})}} = {\frac{\rho_{Ag}}{\sigma_{Ag} \times SER \times M_{Ag}} \times 100}$

wherein a σ_(Ag) the specific conductivity of silver (equal to6.3×10⁷S/m), σ_(Coat) is the specific conductivity of the Ag coating andρ_(Ag) is the density of silver (1.049×10⁷ g/m³).

Measuring the Instability Index

The stability of a silver ink/flushing liquid mixture was measured usingmultiple light scattering coupled with a vertical scanning to monitorthe dispersion state of the mixture. Acceleration of the sedimentationphenomena can be induced for example by fast centrifugation of thesample during the measurement.

A commercial available apparatus is for example a Lumisizer® from LUMGmbH. The samples were measured during 4 hours at 3000 rpm with 880 nmradiation. An instability index provided by the Lumisizer® rangesbetween 0 and 1, wherein the instability increases from 0 to 1.

Viscosity Measurements

Unless otherwise provided, viscosities were measured at 25° C. at ashear rate of 1000 s⁻¹ using a commercially available viscometer forexample as a DHFR-2 Rheometer (double wall ring) from TA Instruments.

Example 1

Preparation of the Silver Ink AgInk-01

450 g of silver oxide (commercially available from Umicore) was added toa mixture of 875 g of ethanol and 517 g of 2-pyrrolidone while stirring.The obtained predispersion was then further stirred at 23° C. for 15hours.

Then, 2.8 g of DSC-01 was added to the mixture followed by the additionof 73 g of formic acid (10.0 mL/min) while stirring and keeping thetemperature at 23° C. After the addition of the formic acid, the mixturewas further stirred for another 15 hours at 23° C.

The dispersion was then concentrated by evaporation of the organicsolvent to obtain a concentrated silver nanoparticle dispersion with asilver content of approximately 45 wt %.

The silver ink AgInk-01 were then prepared by mixing 44 wt % of theconcentrated silver nanoparticle dispersion with 50 wt % of2-phenoxyethanol, 6 wt % propylenecarbonate and 10 wt % n-butanol, allwt % based on the total weight of the silver ink.

Preparation of the Flushing Liquids Flush-01 to Flush-12

The flushing liquids Flush-01 to Flush-12 having a composition as shownin Table 3 have been prepared by mixing the different solvents at roomtemperature.

TABLE 3 Ingredients Flush- (wt %) Flush-01 Flush-02 Flush-03 Flush-04Flush-05 06 2-phenoxyethanol 69 64 59 54 50 50 Propylene 21 16 11 6 50 —carbonate n-butanol 10 20 30 40 0 — 2-pyrrolidone — — — — — 50 ethanol —— — — — — Flush- Flush-07 Flush-08 Flush-09 Flush-10 Flush-11 122-phenoxyethanol 50 100 — — — — Propylene 25 — 100 — — — carbonaten-butanol — — — 100 — — 2-pyrrolidone 25 — — — 100 — ethanol — — — — —100

The viscosity of each prepared flushing (determined as described aboveand measured at 25° C. at a shear rate of 1000 s⁻¹) is presented inTable 4, as well as, the instability index (determined as describedabove) for a full series of Ag ink:flushing liquid mixtures. Theinstability index has been measured for several mixtures of the silverink AgInk-01 and the flushing liquids Flush-01 to Flush-06 having awt/wt ratio as shown in Table 4. In Table 4 the mixture 1:99 stands fora mixture comprising 1 wt % silver ink and 99 wt % flushing liquid, etc.

TABLE 4 Flushing viscosity instability index liquid (mPa · s) 1:99 10:9030:70 50:50 Flush-01 7.3 0.09 0.13 0.08 0.09 Flush-02 6.2 — 0.15 0.100.09 Flush-03 5.4 — 0.22 0.17 0.12 Flush-04 4.9 0.33 0.36 0.23 0.17Flush-05 5.0 0.92 0.18 0.25 0.30 Flush-06 17.0 0.01 0.03 0.04 0.06Flush-07 9.4 0.03 0.13 0.17 0.24 Flush-08 20.4 0.00 0.02 0.03 0.03Flush-09 2.6 1.00 1.00 0.64 0.61 Flush-10 2.7 1.00 1.00 0.93 0.64Flush-11 13.3 1.00 0.08 0.08 0.015 Flush-12 0.9 1.00 1.00 1.00 1.00

From the results of Table 4 it is clear that mixtures of the silverinkjet ink AgInk-01 and the flushing solutions, which do not contain atleast 25 wt % of 2-phenoxy ethanol (Flush-09 to Flush-12), are notstable.

The Flushing solution, which contains 100 wt % 2-phenoxy ethanol, has aviscosity higher than 20 mPas·s (25° C. at a shear rate of 1000 s⁻¹).Such a high viscosity may, depending on the printheads and printersystems, result in difficulties when introducing and flowing such highviscous liquid through the printer tubings and printhead, for instancebefore loading the Ag inkjet ink into the system or during a cleaningprocess.

The best results, combining a low viscosity and a high stability areobtained with Flush-01, comprising at least 25 wt % of 2-phenoxy ethanoland from 5 wt % to 20 wt % of butanol, all based on the total weight ofthe Flushing liquid. Thus, when loading the Ag inkjet ink into theprinting system or during a cleaning process, no sedimentation of Agnanoparticles will occur inside the printhead or printing system whichwill increase the lifetime of the printhead and also allows for a betterjetting performance of the Ag inkjet ink.

1-15. (canceled)
 16. An inkjet ink set comprising: a silver inkjet ink;and a flushing liquid; wherein the flushing liquid includes at least 25wt % of 2-phenoxy ethanol based on a total weight of the flushingliquid.
 17. The inkjet ink set according to claim 16, wherein theflushing liquid further includes a solvent selected from the groupconsisting of propylene carbonate, n-butanol, and 2-pyrrolidone.
 18. Theinkjet ink set according to claim 16, wherein the flushing liquidfurther includes between 5 and 20 wt % of n-butanol based on the totalweight of the flushing liquid.
 19. The inkjet ink set according to claim16, wherein a viscosity of the flushing liquid is lower than 15 mPa·s,measured at 25° C. at a shear rate of 1000 s⁻¹.
 20. The inkjet ink setaccording to claim 16, wherein the silver inkjet ink includes silvernanoparticles
 21. The inkjet ink set according to claim 16, wherein thesilver inkjet ink includes a liquid carrier
 22. The inkjet ink setaccording to claim 21, wherein the liquid carrier is a high boilingsolvent selected from the group consisting of 2-phenoxy ethanol,4-methyl-1,3-dioxolan-2-one, n-butanol, 1,2 propanediol,4-hydroxy-4-methyl pentan-2-one, pentan-3-one, 2-butoxy-ethanol,1-methoxy-2-propanol and mixtures thereof.
 23. The inkjet ink setaccording to claim 21, wherein the liquid carrier includes at least 25wt % of 2-phenoxyethanol and between 5 and 20 wt % of n-butanol.
 24. Aninkjet printing method of preparing a conductive layer or patterncomprising the steps of: providing an inkjet ink set including thesilver inkjet ink and the flushing liquid as defined in claim 16;cleaning a printhead with the flushing liquid; jetting the silver inkjetink on a support to form a silver layer or a pattern on the support; andcuring the silver layer or the pattern to form the conductive layer orthe pattern.
 25. The inkjet printing method according to claim 24,wherein the step of curing is performed at a temperature below 150° C.26. The inkjet printing method according to claim 24, wherein the stepof curing has a curing time of less than 30 minutes.
 27. The inkjetprinting method according to claim 24, wherein the step of curing isperformed with near infrared radiation.